An abundance of habitable planets?

Data from the Kepler space telescope suggest that about a fifth of all sunlike stars in our galaxy are accompanied by potentially habitable Earth-size planets. The Milky Way might be home to 25 billion rocky worlds with liquid water. Read related article.

The habitable zone, or "Goldilocks zone," is the orbital distance from a star in which a planet's atmosphere allows water to exist in liquid form at the planet's surface. So far, that is the only known habitat supporting life.

Some scientists theorize that on cold planets outside the habitable zone, life might exist where water is kept liquid through tidal heating or radioactive decay.